Hi Chipp - the offset mechanism in the geometry library that Moi uses has a lot of limitations in it and it gets particularly confused when you try to thicken a surface made up of multiple pieces that are not totally smooth to one another. When that happens the offsets do not themselves naturally meet up and the surfaces have to be extended in order to meet. It's that extension process that is rather prone to failure.

It would work better if you were to have a fully smooth object, like using something like Construct > Blend to build the corner piece rather than having a corner piece that is not smooth to the surrounding surfaces. Your current corner piece is only smooth at the outside edges of it, it kind of sags down a bit in the middle zone leaving a fairly sharp crease in those spots:

The offset of multiple joined surfaces with sharply meeting edges is a much more complex type of operation than an offset of pieces that are all smooth to one another, and also it's even more difficult if things are kind of relatively close to being smooth to each other because when surfaces that are nearly smooth to each other are extended it is then difficult to get a good crisp intersection between the extensions, there is sort of more like a "zone of overlapping area" rather than a crisp intersection.

So it basically does not work very well to try and offset a joined surface that has a structure like you have here with shallow meeting sharp pieces in it.

It would probably be better to build a blocky object to start with and then use filleting to put in all the round parts rather than trying to manually construct the rounded pieces.

Blend can be your friend in this case. I used G1 for the profile curves and swept the sides of the model and then just did a set of G2 surface blends to create the inner and outer corners. the remainder of the operations were the Planar command followed by Join to a watertight solid. I can put a short vid up if it helps.

Here it is, no audio but it should be easy enough to follow along, under 7 minutes which is comparatively short for me. I must have picked a wrong edge to create one of the planes late in the video because it wouldn't get watertight, so I just deleted and tried again. It's rough and some things could be streamlined but I figure it helps to leave those sort of things in there to show mistakes and how to work through them. I hope it's helpful.

No particular reason, extrude works fine on the walls and will leave you with a solid. I guess it goes to show there's more than one way to come come up with the desired result in MoI. If you need to create walls that do not follow a linear path Sweep is one way to get that result while using Blend (shown in green) to connect those walls with desired continuity, as shown here in this image. As far as the solid issue goes I think I must have not selected the correct edge to create the upper plane, I try to remember to hide the curves when doing those sort of surfacing operations so I am able to pick the correct surface edges, that's my guess anyway.